THE CATHODE PREPARATION CHAMBER FOR THE DC HIGHCURRENT HIGH POLARIZATION GUN∗

O. Rahman† , I. Ben-Zvi, D. Gassner, A. Pikin, T. Rao, E. Riehn, B. Sheehy, J. Skartika,E. Wang, Q. Wu, BNL, Upton, NY 11973, USA.

AbstractA compact cathode preparation chamber for the high

current high polarization gun for the proposed eRHICproject has been designed and assembled at BrookhavenNational Laboratory. This preparation chamber will beused to activate GaAs photocathodes to be used in theGatling gun. The chamber is capable of achieving XHVon a consistent basis. Bulk GaAs samples were activatedin this chamber with standard QE for the respective wave-length. In this paper, we discuss the design of this vacuumsystem, the heat cleaning and the activation procedure forthe GaAs sample which will eventually be followed for theGatling gun.

INTRODUCTIONThe High Current High Polarization Electron Gun is one

of the most important research projects for the proposedeRHIC project [1]. The average current is required to be 50mA and the polarization requirement is more than 80%.Forweekly exchange of cathodes, given that two guns are used,one cathode has to provide 50 mA of average current for 85hours, which yields to 15300 C of charge. The maximumreported charge extracted is 1000 C, which is a order ofmagnitude lower than the requirement [2]. To solve thisproblem, 50 mA of current can be distributed over 20 pho-tocathodes such that each cathode has to provide 2.5 mAof current for 85 hours, yielding 765 C of charge. This so-lution of combining 20 bunches to one common beam axissolves the problem of charge limitation without sacrificingthe high average current requirement. The high polariza-tion can be obtained using Superlattice GaAs which is ableto provide polarization greater than 80% in a consistent ba-sis [3]. For the R&D and “proof of principle” experiment,bulk GaAs will be used.

DESIGN OF THE CATHODEPREPARATION CHAMBER

The cathode preparation chamber is a compact designincluding 3 crosses, two of them are 6-way crosses and theother one is a standard 4 way cross. These three crossesare stacked on top of each other with the 4-way cross at thebottom. The cathode holder is moved up and down usinga magnetic manipulator. One NEG pump and one turbopump is attached to the bottom 4-way cross. The middle

∗Work supported by Brookhaven Science Associates,LLC under con-tract no. DE-AC02-98CH10886 with the U.S. Department of Energy† orahman@bnl.gov

NEG 1

NEG 2

Anode Cs source

Gauge

Bellow tomove filament

Filament

Turbo pump

Window

Ion pump

Figure 1: Cross sectional view of the cathode preparationchamber.

cross contains the tungsten filament, another NEG pump,one Ion pump and a view port. The top-most cross includesthe Cs source, a pick up anode, a variable leak valve forOxygen, an Extractor gauge to measure low pressure and awindow through which laser shine the sample. A cross sec-tional view of the assembly is shown in Fig. 1 and the frontview of the actual system is shown in Fig. 2. The vacuumand heat cleaning components are described in detail in thefollowing sections.

VACUUM COMPONENTS AND BAKEOUT PROCEDURE

The bake out procedure is extensive since activatingGaAs requires XHV i.e low 10-11 Torr scale vacuum. Thechamber at 200◦C for approximately a week. Heater tapesare used to wrap the chamber and K-type thermocouplesare used in 8 different locations of the chamber to make

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Figure 2: Front view of the cathode preparation chamber.

Table 1: Vacuum Pumps used in the Prep. Chamber

Pump Pumping speed location in the chamber

Ion pump 50 l/s Middle crossNEG 1 80 l/s Bottom crossNEG 2 80 l/s Middle cross

sure that the temperature is uniform over the entire cham-ber.Different pumps that are installed in this system is de-scribed in Table 1.The NEG pumps are Capacitorr D400units from SAES getters and the Ion pump is a standardGamma Vacuum one.

Temperature data is saved to a computer using anOmega USB data acquisition module. The temperature ofthe chamber is increased gradually from room temperatureto 200◦C in 50◦C increments over the course of two days.

One AxTRAN ISX2 gauge, a hot cathode extractor typegauge, is used to monitor pressure data. The NEG pumpsare activated twice during the bakeout cycle, once the tem-perature of the system is 100◦C and then right before thecooling of the system begins. Every other component of thesystem is also degassed during the heating cycle when thesystem is at 200◦C including the filament, the Cs source,the cathode holder and the gauge.A typical Pressure Vs Time graph is show in Fig. 3. While

cooling down, the system is also brought down graduallyfrom 200◦C to 80◦C to room temperature. While the sys-tem is at 80◦C, the RGA is degassed.

HEAT CLEANING AND ACTIVATIONBefore GaAs is activated, the surface of the sample

needs to be cleaned either by conventional heat cleaningor atomic Hydrogen cleaning. We use a commerciallyavailable Tungsten filament from a 250W light bulb toheat up the sample by radiation to a desired temperature.The heat cleaning setup is shown in Fig. 4. The filament

Figure 3: Pressure of the chamber during a bake-out proce-dure.

is inserted into the chamber using a feedthrough which isconnected to a bellow that is compressible and the positionof the filament can be adjusted.It should be noted that the vertical position of the cathode

NEG 1

NEG 2

Anode Cs source

Gauge

Bellow tomove filament

Filament

Turbo pump

Window

Ion pump

Figure 4: Schematic diagram of the heat cleaning proce-dure.

and the position of the filament has to be exact in order tohave the right temperature on the surface of the sample.Meticulous calibration tests were performed to measurethe surface temperature of the GaAs as a function ofthe driving current through the filament with a fixedvertical height and fixed filament position. For our heatcleaning procedure, we use the calibration curve to obtain

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a temperature of 580◦C on the surface of the GaAs.

After the heat cleaning is done for 45 minutes, it takesapproximately 2 hours for the sample to come back downto room temperature (also obtained from the calibration ex-periment). For activation, we use a standard evaporation Cssource from SAES getters. The laser is a 650 nm diode CWlaser. A sample activation curve is shown in Fig. 5.

Figure 5: Typical activation curve.

Typically the first real signal is measured after 350 sec-onds and the Cs peak is seen after 500 seconds. This acti-vation process is a co-deposition process where the Cs andOxygen are applied to the surface simultaneously after thesignal has reached the Cs peak. Multiple activations wereperformed using this technique and the QE ranges between6% to 8% for 650 nm light.

CONCLUSION AND FUTURE PLANA compact prototype preparation chamber to activate

GaAs photocathodes is assembled and tested. The cham-ber has shown to be XHV achievable and bulk GaAs sam-ples were activated in the chamber with standard QE for650 nm light. This chamber is scheduled to be installedon the storage chamber and a two cathode test is scheuledto be performed early next year. Eventually, this chamberprototype will be used to activate Superlattice GaAs pho-tocathodes for the full operation of the High Current HighPolarization Gun.

REFERENCES[1] X. Chang ,et al. ”A multiple cathode gun design for the eR-

HIC polarized electron source,” Proc. of PAC2011, WEP263,p.1969 (2011).

[2] J. Grames ,et al. ”Charge and fluence lifetime measure-ments of a dc high voltage GaAs photogun at high averagecurrent,” Physical Review Special Topics-Accelerators andBeams 14.4 (2011) 043501.

[3] T. Maruyama et al., ”Systematic study of polarized electronemission from strained GaAs GaAsP superlattice photocath-odes,” Applied Physics Letters 85 (2004) 2640.

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